1. Field of the Invention
Our invention is a type of instrument known as a two-fluid tiltmeter, which is used to measure relative vertical displacement between the ends of a known baseline. From this measurement, regional tilt may be inferred or determined. The benefits of accurately determining regional surface tilting with our instrument are many and include, but are not limited to, the following: to assist in predicting earthquakes; to measure surface uplift and subsidence from geophysical forces (volcanoes, geothermal fields) and industrial activity (mining, petroleum and ground water pumping); to monitor the stability of salt domes that could be used for storage of oil or radioactive waste; and to determine the degree of tilting in areas where sensitive instruments that require stable bases are being used (i.e., astronomical observatory, lunar laser ranging observatory).
2. Description of the Prior Art
At the present time, vertical displacement is generally determined by using gravity meters, precision levels (spirit levels and/or bubble tiltmeters), and single-fluid tiltmeters. The use of a gravity meter to infer vertical displacement assumes that the material beneath the instrument has a constant density; this may be a poor assumption, particularly in an active volcanic region. The use of spirit levels is subject to many systematic errors, and great care and expense is required to measure vertical displacements over kilometer-long baselines to a precision approaching a millimeter (inferred average tilt of a microradian). The use of bubble tiltmeters, which are short baseline instruments, is significantly limited by their great sensitivity to surface conditions near the piers.
Many of the problems associated with gravity meters, spirit levels, and bubble tiltmenters are overcome by using a medium-length baseline single-fluid tiltmeter, which consists of a horizontal tube connecting two vertical reserviors mounted on separated piers. A major advantage of this type of prior art system is that localized tilting at the piers produces only second order errors in regional tilt determinations. There are, however, major disadvantages in using this system. Errors in meter readings occur when using baselines of considerable length (over 10 meters) because of variations in the fluid's density caused by temperature fluctuations along the measurement path. To reduce these errors, it is usually necessary to resort to the impractical expedient of deploying the tiltmeter in a horizontal position and burying it.
Our invention is designed to resolve these problems associated with the single-fluid tiltmeter. As explained hereafter, our two-fluid tiltmeter may be used over long baselines (i.e., 1 kilometer) without burial or precise horizontal deployment and provides the data necessary to correct for thermally induced errors.